Each PI in the Program Project will utilize the murine heterotopic cardiac graft model in one or more of their Specific Aims. This in vivo model of chronic graft rejection is highly versatile and invaluable as an experimental tool. Unfortunately, it is also technically demanding and resource intensive. For these reasons, it is offered through a Core facility staffed with microsurgical experts. The Specific Aims of the Core are: 1) To procure the mice necessary for the proposed studies and maintain them in a manner that guarantees their health and safety. 2) To perform heterotopic cardiac grafts in numbers sufficient to support the needs of the investigators with minimal technical complications. 3) To treat graft recipients with immunosuppressive agents that prevent acute rejection and permit the development of chronic rejection histopathology (principally anti-CD4 mAb and gallium nitrate). 4) To monitor the graft recipients post-transplant for quality of graft function, and to provide high-quality post-transplant animal care. 5) To assist investigators in surgical procedures necessary for the recovery of grafts or other required tissues.